Nonequilibrium chemical processes

In nonequilibrium systems, chemical processes spontaneously alter the composition or phase of the system until equilibrium is attained. Simple systems, such as a mixture of sodium chloride and water, attain equilibrium quickly, whereas complex systems may reach equilibrium only after decades or eons. 

Eliasson, B. and Kogelschatz, U. (1991) Nonequilibrium volume plasma chemical processing, IEEE Trans. PI. Sci. 19, 1063-77. 

An actual chemical process as it might occur under either equilibrium or nonequilibrium conditions in a chemical reactor. 

The physicochemical stage includes the chemical processes in electron excitation states, as well as the chemical transformations of the active intermediates under nonequilibrium conditions. These are the predissociation and the ion-molecular reactions that take about 1013 s the recombination of positive ions with thermalized electrons (1CT12-10 10s) and the electron-solvation reactions (10 12-10-1° s). Thus, the physicochemical stage lasts from 1CT13 to 10-I0s. 

Now, we will consider a nonequilibrium chemical process in a polymeric system described by equations of linear thermodynamics ... 

Chemical process rate equations involve the quantity related to concentration fluctuations as a kinetic parameter called chemical relaxation. The stochastic theory of chemical kinetics investigates concentration fluctuations (Malyshev, 2005). For diffusion of polymers, flows through porous media, and the description liquid helium, Fick s and Fourier s laws are generally not applicable, since these laws are based on linear flow-force relations. A general formalism with the aim to go beyond the linear flow-force relations is the extended nonequilibrium thermodynamics. Polymer solutions are highly relevant systems for analyses beyond the local equilibrium theory. 

Time scales, which are characteristic of various types of physical and chemical processes, are significantly different. This makes it possible to categorize thermodynamic processes in the system into internal and exter nal processes. Therefore, the first step in considering nonequilibrium thermodynamic processes is to establish the hierarchy of the processes along the time scale. 

The simplest case of a genuine nonequilibrium system is one with uniform and time constant temperature and pressure. Examples include open or closed systems that consist of low intensive ( sluggish ) chemical processes where molecules of the initial reactants, final products, and intermediates may all be considered thermalized. 

This specific feature of the stationary state of chemical systems that undergo their evolution via an arbitrary combination of only monomolec ular (or reduced to monomolecular) transformations, as well as transforma tions that are linear in respect to the intermediates, is of practical importance to simplify the analysis of complex stepwise chemical processes with the use of methods of nonequilibrium thermodynamics. 

Whereas mutual diffusion characterizes a system with a single diffusion coefficient, self-diffusion gives different diffusion coefficients for all the particles in the system. Self-diffusion thereby provides a more detailed description of the single chemical species. This is the molecular point of view [7], which makes the selfdiffusion more significant than that of the mutual diffusion. In contrast, in practice, mutual diffusion, which involves the transport of matter in many physical and chemical processes, is far more important than self-diffusion. Moreover mutual diffusion is cooperative by nature, and its theoretical description is complicated by nonequilibrium statistical mechanics. Not surprisingly, the theoretical basis of mutual diffusion is more complex than that of self-diffusion [8]. In addition, by definition, the measurements of mutual diffusion require mixtures of liquids, while self-diffusion measurements are determinable in pure liquids. 

As mentioned above, the nonequilibrium radiation code NEQAIR is employed for prediction of ultraviolet emission from the DSMC flow field solutions. The modeling of ultraviolet emission with this code is discussed for nitric oxide in Ref. 84 and for atomic oxygen in Ref. 87. A common assumption made in using the NEQAIR code is that a quasisteady state (QSS) exists for the number densities of the electronically excited species. The assumption requires that the time scale of chemical processes is much smaller than the time scales for diffusion and for changes in overall properties. Under these conditions, the local values of temperatures and ground state species number densities obtained from the DSMC computation may be used to compute the populations of the electronically excited states. 

The delay in using molecular modeling in chemical process industry stems from many challenges, including the inherent complexity of nonequilibrium processes, the lack of a rigorous nonequilibrium statistical mechanics theory, the lack of experimental techniques with nanometer spatial and short temporal resolution that can be confronted with molecular models, and the inherent multiscale complexity of industrial processes. Despite these obstacles, molecular modeling is nowadays clearly an integral part of multiscale and nanoscience research,f ° as it can handle complex... 

Molecular modeling for nonequilibrium processes, such as heterogeneous catalysis and growth of advanced materials, is a relatively new tool in the chemical process industry. The advent of algorithms and advances in computer power on the one hand, and the development of coarse graining and multiscale... 

Jardine, P. M. 1991. Modeling nonequilibrium reactions in inorganic solute in soil columns, p. 255-279. In D.L. Sparks and D.L. Suarez (ed.) Rates of soil chemical processes. SSSA Spec. Publ. 27. SSSA, Madison, WI. 

Eliasson. B. and Kogcischatz, U. Nonequilibrium volume plasma chemical processing IEEE Trans. Pla.smu Set,. 1991,19, 1063-1077... 

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